Plastics have become an indispensable part of modern life. Their versatility in terms of physical properties, formability and cost has made plastics the material of choice for many product applications. The vast majority of plastics materials are made from depleting petroleum fossil resources providing a strong driver to not only reduce their consumption but to extend the use of these materials by recycling. Plastics have also become a symbol of disposability and hence an inevitable object of concern given heightened public awareness of environmental issues providing strong drivers to recycle or re-use post-consumer plastic materials and avoid disposing of them in landfills or the environment. During the melt processing of plastics, it is inevitable that a certain percentage of the material will not be directly converted into usable end product. Scrap material can be generated during processing equipment startup (purge material), operation (sprues, flash, runners and product rejects) and shutdown. For some plastics however generated scrap material can be recycled directly back into the melt process aiding both the economics and environmental impact of the overall product manufacturing process. Prior to recycling of the material, it is typically ground (termed regrind), chopped or crushed to a uniform and specific particle size and in some cases it is necessary to mix it with unprocessed or virgin plastic material to make a useful product formulation or compound. It is also desirable to be able to use or recycle the plastic from products after their useful lifetime using the same approach of grinding up the used plastic products to make a regrind material and combining that with virgin material as needed in an extruder to re-process it into new products.
Polyvinylchloride (PVC) is one of the most important commodity plastics and is used in a very wide range of applications form rigid to flexible applications. In order to deliver PVC products to this wide array of end uses it is one of the most highly formulated plastics in use and a PVC part can contain up to 50% or more of other materials. These other materials can be collectively referred to as additives and include everything from mineral fillers, plasticizers, processing aids, UV stabilizers oxidative stabilizers etc. PVC has the disadvantage of having relatively low thermal stability and this is further complicated by the high levels of additives used to make PVC broadly useful in applications. Thermal stability in general relates to the process whereby through high temperature, high pressure, oxidation or mechanical stress, the long chains of a polymer begins to break and react with one another thereby changing the properties of the polymer. Since thermoplastic materials are usually formed into products by the application of heat, pressure and/or mechanical stress, degradation can pose a serious problem. It is also well known that the reduction in molecular weight of PVC due to thermal processing can negatively impact many key physical properties such as tensile strength, impact resistance, softening temperature and long term aging, many of which are dependent on maintaining a consistent molecular weight. Chemical reactions that can occur in PVC when it is processed for the first time can result in crosslinking and other reactions which alter the melt rheology and make it more difficult to process the second time around. This manifests itself as increased torque in the extruder which extends processing times and can result in lower physical performance of the parts produced.
To prevent thermal degradation from occurring in polyvinylchlorides, additives such as organotin mercaptides/sulfides or metal carboxylates are usually added. The metal carboxylates are mixtures based on salts of aliphatic (oleic) or aromatic (alkylbenzoic) carboxylic acids usually with combinations of barium/zinc or calcium/zinc metals. These additives improve thermal stability by acting directly at the dehydrochlorination initiation site and/or by reacting with the free HCl generated. In the case of the metal carboxylates, reaction with HCl produces chloride salts which can also have a destabilizing effect on the PVC. Therefore co-stabilizers such as polyols, phosphites and epoxy plasticizers are often used along with the metal carboxylates to improve initial color, transparency and long term PVC stability.
For semi-rigid and flexible polyvinylchlorides, plasticizer's are also a major component of the overall product formulation. It has been found that plasticizer type, concentration and oxidative stability (formation of peroxide radicals) all affect the thermal stability of PVC.
The use of PVC recyclates (plastic material that have been thermally processed at least once) to produce products is therefore limited due to the impairment of physical properties caused by melt processing the first time. To avoid simply wasting PVC and to enable recycling, the PVC recyclates are usually combined with virgin unprocessed PVC resin and the level of recyclate that can be used is typically limited to only 5-25% by weight of the total plastic weight in a product formulation before the physical properties of the final product are too impaired to be useful in the targeted application.
Products made from PVC in many cases have long term use in construction (roofing membranes, doors, window frames, decking, partitions, furniture, floor coverings (carpet, tiles) and the like) and automotive and industrial applications and have very high levels of fillers and additives. It is well known that recycling of PVC is both costly and technically challenging but also very desirable from an environmental and sustainability perspective. The approach of recycling PVC products for energy capture is not a desirable solution because of producing undesirable toxic compounds such as dioxins. One current method being promoted for recycling of PVC involves solvent extraction of the PVC polymer from the collected used parts. The need to develop improved technologies to enable the recycling or re-use of PVC is of growing importance as many of the PVC parts used in construction applications which were introduced 20-30 years ago are now reaching the end of their useful life. It is therefore desirable to be able to take post-consumer use PVC products and recycle them via melt extrusion into new products. Before PVC parts can be re-cycled in a melt extrusion process they have to first be converted into a smaller particles that can be fed into an extruder. This is typically accomplished by grinding up the used parts to make a ‘regrind” material. In many cases the percentage of recycled PVC that can be used in a given product formulation will be restricted by the final product physical properties. Reduced property performance of PVC products made using regrind or recylate PVC is well known in the industry to be a major problem.
One method for improving the value of and extending the usable range of recycled plastics (such as recycled PVC) is to mix them with additives which can maintain or improve their overall performance in product formulations. Therefore a need exists to develop functional additives which when added to recycled plastics can enhance their material properties and minimize physical property loss with higher loadings of recyclates.